7.2 Example 2

A block with a mass of 5 kg, is above a 30 degrees inclined plane. A string keep the block in rest (see image). If there isn’t friction, which is the value of the stress in the string?.

Solution with FisicaLab

Select the Static group and, inside this, the Particles module. Erase the content of the chalkboard and select the SI system. And add two elements Oblique force, one element Block above an inclined plane to the right and one element Stationary reference system, to make the free body diagram of the block. As show the image below:

To the Stationary reference system:

g

9.81

To the element Block above an inclined plane to the right write:

m

5

ang

30

To the element Oblique force, that correspond to the normal force, its angle is 60 degrees (if the plane is inclined 30 degrees, the normal force is inclined 60 degrees). Then write:

f

n

ang

60

To the Oblique force that correspond to the stress in the string, the angle is the 30 degrees (this force is parallel to the plane). Then write: